Research on improving detection sensitivity to acoustic based on coherent-OTDR distributed fiber-sensing system

Abstract

Coherent-OTDR technology is one of acoustic distributed fiber-sensing systems. Because of the advantages of anti-electric magnetic field interference, anti-corrosion and flexibility, it has been attracting more and more interest. Because the sound pressure is weak, the strain generated on the fiber is tiny and the sensitivity of the sensing system is low. Although many research has been made on expanding measuring distance and improving response frequency, the acoustic signals in the experiments are always replaced by PZT's mechanical stretching. In this work, a device for increasing sensitivity for acoustic in the passive acoustic detection system based on coherent optical time domain reflection (C-OTDR) is promoted. A way of improving sensitivity partly based on a thin-walled corrugated tube was promoted. The thin-walled corrugated tube was used as the element to transmit the energy of acoustic into the vibration of fiber. In section 2, a mathematical model of sensing based on corrugated tube was established. Theoretical result shows that the vibration of fiber is mainly caused by the tube movement along the axis direction. And it also shows the linear relationship between the vibration and the sound pressure. The sensitivity of the improved sensing devices is calculated and a computational formula for sensitivity calculating are also given. In section 3, the C-OTDR acoustic distributed fiber-sensing systems are set up. Fiberring and three types of thin-walled corrugated tubes are used for acoustic sensing. The minimum detection sound pressure level reaches 60.1 dB and the phase sensitivity reaches 2.975 rad/Pa. The experimental phase sensitivity of different sensing devices with different parameters change similarly to the theory results. The experimental results show that the way of improving sensitivity and the mechanical model for calculating sensitivity are effective. This research provides theoretical and experimental basis for further development of distributed optical fiber sensing.